Alpha acids (or humulones) have the formula AA: ##STR1## wherein R is isopropyl, isobutyl, or secondary butyl.
Alkali metal humulates have the formula H: ##STR2##
Alkali metal tetrahydrohumulates have the formula THH: ##STR3##
Tetrahydroalpha acids (or tetrahydrohumulones) have the formula THAA: ##STR4##
Isoalpha acids (or isohumulones) have the formula IAA: ##STR5## and are not involved according to the present invention.
Tetrahydroisoalpha acids (or tetrahydroisohumulones) have the formula THIAA: ##STR6##
Tetrahydroisohumulates have the formula THIH: ##STR7## it being recognized that keto-enol tautomerism exists.
Isohumulates have the formula IH: ##STR8## and are not involved according to the present invention.
The bitterness of beer is due to the presence of isoalpha acids, which are currently used in both their reduced and unreduced forms, and ordinarily in the form of an isohumulate or reduced isohumulate. The unreduced form is made by isomerization by boiling of the alpha acids naturally present in hops, or by catalytic isomerization utilizing magnesium or calcium ions, all as well known in the art. Tetrahydroisoalpha acids or salts thereof have been made by oxidative conversion of beta acids to tetrahydroalpha acids (Worden U.S. Pat. No. 3,923,897), and isomerization of these hydrogenated alpha acids to tetrahydroisoalpha acids. More recently Hay (U.S. Pat. No. 5,013,571) has described a process for the "simultaneous" catalytic isomerization and hydrogenation of isoalpha acids to tetrahydroisoalpha acids, as well as the hydrogenation of previously-isomerized isoalpha acids. Hay (U.S. Pat. Nos. 5,013,571 and 5,013,572) has also described a method of steam stripping unwanted estery aromas from the hydrogenated isoalpha acids produced by his process.
Todd (U.S. Pat. No. 4,778,691) describes the separation of unwanted impurities into an aqueous phase from an organic phase of unreduced and/or reduced alpha and isoalpha acids. Todd (U.S. Pat. No. 4,647,464) describes the separation of alpha and beta acids from a carbon dioxide hop extract, providing an alpha acid fraction which is essentially devoid of impurities and which may be isomerized by heat alone to isoalpha acids without the formation of isomerization by-products, by using less than one mole of base per mole of alpha acid in the separation step. Such an alpha acid fraction is an excellent starting material for use according to the present invention.
The general background of the art is well described in the foregoing series of patents. Worden shows that tetrahydroalpha acids can be made from beta acids and then isomerized to tetrahydroisoalpha acids. Alternatively, alpha acids are isomerized to isoalpha acids, which in turn are hydrogenated, as shown by Hay, even when the two successive steps are carried out in the same medium (Hay, '571, Col. 2, lines 33-37, and confirmed in Example 9 hereof).
The art has not been able to utilize alpha acids as a source of tetrahydrohumulates, with subsequent isomerization to tetrahydroisohumulates. Verzele, (Bulletin des Societes Chimiques Belges 68. pp 315-324 and 476-583 (1959)) found that, upon hydrogenation of alpha acids at a pH up to 8.1 on the alkaline side, substantial amounts of quinone formation occurred. Verzele used a minimum of 2.2 moles of hydrogen per mole of alpha acids, which also resulted in perhydrogenation or over-reduction under his conditions. This may be attributed to the partial hydrogenation of keto groups on the alpha acid molecule. The formation of humuloquinones HQ: ##STR9## depends upon the presence of the six-membered ring present in alpha acids, and they (HQ) cannot be formed from a compound having the five-membered ring of the iso acids. This is why Hay could produce tetrahydroisoalpha acids by hydrogenation of isoalpha acids without quinone formation, even though he employed hydrogen pressures of up to 2,000 psig and a pH as low as 5 to 7 (Hay '571, Col. 9).